Communication by optical signal is a known technology that uses the propagation of light to transmit information over a communication channel between two remote points. It is already used to exchange information between satellites or between a satellite and a fixed terminal on the earth, and is commonly used in terrestrial fiber-optic telecommunication networks. An optical signal, for example a laser beam modulated by effective data, is sent from a transmitter terminal to a receiver terminal. In general, the communication channel of an optical transmission device can be empty space (the atmosphere, space), the marine environment, an optical guide or any other medium that is transparent to light. The conditions for transmitting an optical signal through these different media may vary over time or space. These disturbances cause a deterioration in the quality of the optical signal received by the receiver terminal (attenuated signal, random phase) and are liable to alter the data being transmitted.
As shown in FIG. 1, a satellite S is fitted with a transmitter terminal TE that sends digital data to a receiver terminal TR located on the earth T using a primary optical signal S1. The terrestrial atmosphere may disturb the propagation of the signal between the two terminals and thereby cause transmission errors in the digital data. Various techniques have been developed to adapt the optical transmission to the disturbances P on the communication channel. Document FR2957214, which describes an optical transmission method using laser signals in which an encoding rate of the digital data sent by a laser signal is adapted as a function of the propagation conditions of the laser beam, is in particular known. To take account of the effect of these disturbances P on the optical transmission, the use of a secondary optical signal S2, sent by the receiver terminal and received by the transmitter terminal, is known. The propagation conditions of the optical signal, characterized by the secondary optical signal S2 received by the transmitter terminal, make it possible to adapt the encoding rate, and therefore the effective rate, in real time. A low encoding rate is used for highly disturbed optical transmission, so the effective digital data in the signal sent has a greater degree of redundancy.
The adaptation of the effective rate according to known solutions only enables transmission to be maintained for disturbances of moderate amplitude or, when it relates to interlacing, for significant disturbances of limited duration. If significant attenuation occurs, or attenuation occurs over an excessively long period in relation to the foreseeable interlacing periods in practice, these known solutions do not provide a satisfactory effective rate. It is therefore desirable to use an optical transmission method that makes it possible to adapt and optimize the effective rate transmitted over an extended range of disturbances on the transmission channel, while enabling fine tuning of adaptation of the effective rate within this range.